Fault current limiting HTS cable and method of configuring same

ABSTRACT

A cryogenically-cooled HTS cable is configured to be included within a utility power grid having a maximum fault current that would occur in the absence of the cryogenically-cooled HTS cable. The cryogenically-cooled HTS cable includes a continuous liquid cryogen coolant path for circulating a liquid cryogen. A continuously flexible arrangement of HTS wires has an impedance characteristic that attenuates the maximum fault current by at least 10%. The continuously flexible arrangement of HTS wires is configured to allow the cryogenically-cooled HTS cable to operate, during the occurrence of a maximum fault condition, with a maximum temperature rise within the HTS wires that is low enough to prevent the formation of gas bubbles within the liquid cryogen.

RELATED APPLICATION(S)

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 11/688,809, filed Mar. 20, 2007, now U.S. Pat. No.7,902,461, and entitled “Fault Current Limiting HTS Cable and Method ofConfiguring Same”, which is herein incorporated by reference.

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No. 11/673,281, filed 9 Feb. 2007, andentitled “Parallel Connected HTS Utility Device and Method of UsingSame”, which is herein incorporated by reference.

This application claims priority to U.S. patent application Ser. No.11/688,802, filed 20 Mar. 2007, and entitled “Parallel Connected HTS FCLDevice”, which is herein incorporated by reference.

This application claims priority to U.S. patent application Ser. No.11/688,817, filed 20 Mar. 2007, and entitled “HTS Wire”, which is hereinincorporated by reference.

This application claims priority to U.S. patent application Ser. No.11/688,827, filed 20 Mar. 2007, now U.S. Pat. No. 7,724,482, issue date:May 25, 2010, and entitled “Parallel HTS Transformer Device”, which isherein incorporated by reference.

TECHNICAL FIELD

This disclosure relates to HTS devices and, more particularly, to HTSdevices configured to operate as fault current limiting devices.

BACKGROUND

As worldwide electric power demands continue to increase significantly,utilities have struggled to meet these increasing demands both from apower generation standpoint as well as from a power delivery standpoint.Delivery of power to users via transmission and distribution networksremains a significant challenge to utilities due to the limited capacityof the existing installed transmission and distribution infrastructure,as well as the limited space available to add additional conventionaltransmission and distribution lines and cables. This is particularlypertinent in congested urban and metropolitan areas, where there is verylimited existing space available to expand capacity.

Flexible, long-length power cables using high temperaturesuperconducting (HTS) wire are being developed to increase the powercapacity in utility power transmission and distribution networks, whilemaintaining a relatively small footprint for easier installation andusing environmentally clean liquid nitrogen for cooling. For thisdisclosure, an HTS material is defined as a superconductor with acritical temperature at or above 30° Kelvin (minus 243° Centigrade),which includes materials such as rare-earth oryttrium-barium-copper-oxide (herein called YBCO);thallium-barium-calcium-copper-oxide;bismuth-strontium-calcium-copper-oxide (herein called BSCCO);mercury-barium-calcium-copper-oxide; and magnesium diboride. YBCO has acritical temperature approximately 90 K. BSCCO has a criticaltemperature of approximately 90 K in one composition and approximately110 K in a second composition. MgB₂ has a critical temperature of up toapproximately 40 K. These composition families are understood to includepossible substitutions, additions and impurities, as long as thesesubstitutions, additions and impurities do not reduce the criticaltemperature below 30° K. Such HTS cables allow for increased amounts ofpower to be economically and reliably provided within congested areas ofa utility power network, thus relieving congestion and allowingutilities to address their problems of transmission and distributioncapacity.

An HTS power cable uses HTS wire as the primary conductor of the cable(i.e., instead of traditional copper conductors) for the transmissionand distribution of electricity. The design of HTS cables results insignificantly lower series impedance, in their superconducting operatingstate, when compared to conventional overhead lines and undergroundcables. Here the series impedance of a cable or line refers to thecombination of the resistive impedance of the conductors carrying thepower, and the reactive (inductive) impedance associated with the cablearchitecture or overhead line. For the same cross-sectional area of thecable, HTS wire enables a three to five times increase incurrent-carrying capacity when compared to conventional alternatingcurrent (AC) cables; and up to a ten times increase in current-carryingcapacity when compared to conventional direct current (DC) cables.

HTS cables may be designed with HTS wires helically wound around acontinuously flexible corrugated former, or they may have multiple HTSwires in a variety of stacked and twisted configurations. In all thesecases the cable may be continuously flexible, so that it can be woundconveniently on a drum for transportation and installed with bends andturns in a conduit or between other power devices. HTS cables may bedesigned with a liquid cryogen in contact with the HTS wires and flowingalong the length of the cable. Liquid nitrogen is the most common liquidcryogen, but liquid hydrogen or liquid neon could be used for lowertemperature superconducting materials like magnesium diboride.

In addition to capacity problems, another significant problem forutilities resulting from increasing power demand (and hence increasedlevels of power being generated and transferred through the transmissionand distribution networks) are increased “fault currents” resulting from“faults”. Faults may result from network device failures, acts of nature(e.g. lightning), acts of man (e.g. an auto accident breaking a powerpole), or any other network problem causing a short circuit to ground orfrom one phase of the utility network to another phase. In general, sucha fault appears as an extremely large load materializing instantly onthe utility network. In response to the appearance of this load, thenetwork attempts to deliver a large amount of current to the load (i.e.,the fault). Any given link in the network of a power grid may becharacterized by a maximum fault current which will flow, in the absencefault current limiting measures. during the short circuit thatprecipitates the maximum fault condition. The fault currents may be solarge in large power grids that without fault current limiting measures,most electrical equipment in the grid may be damaged or destroyed. Theconventional way of protecting against fault currents is to rapidly opencircuit breakers and completely stop the current and power flow.

Detector circuits associated with circuit breakers monitor the networkto detect the presence of a fault (or over-current) situation. Within afew milliseconds of detection, activation signals from the detectorcircuits may initiate the opening of circuit breakers to preventdestruction of various network components. Currently, the maximumcapability of existing circuit breaker devices is 80,000 amps, and theseare for transmission level voltages only. Many sections of the utilitynetwork built over the previous century were built with network devicescapable of withstanding only 40,000 to 63,000 amps of fault current.Unfortunately, with increased levels of power generation andtransmission on utility networks, fault current levels are increasing tothe point where they will exceed the capabilities of presently installedor state-of-the-art circuit breaker devices (i.e., be greater than80,000 amps) both at distribution and transmission level voltages. Evenat lower fault current levels, the costs of upgrading circuit breakersfrom one level to a higher one across an entire grid can be very high.Accordingly, utilities are looking for new solutions to deal with theincreasing level of fault currents. In most cases, it is desirable toreduce fault currents by at least 10% to make a meaningful improvementin the operation of a grid. One such solution in development is a devicecalled an HTS fault current limiter (FCL).

An HTS FCL is a dedicated device interconnected to a utility networkthat reduces the amplitude of the fault currents to levels thatconventional, readily available or already installed circuit breakersmay handle. See High-Temperature Superconductor Fault Current Limitersby Noe and M. Steurer, Supercond. Sci. Technol. 20 (2007) R15-R29. SuchHTS FCLs have typically been configured out of short rigid modules madeof solid bars or cylinders of HTS material which have very highresistance when they are driven over their superconducting criticalcurrent into a resistive state. Unfortunately, such standalone HTS FCLsare currently quite large and expensive. Space is particularly at apremium in substations in dense urban environments where HTS cables aremost needed. Utilities may also use large inductors, but they may causeextra losses, voltage regulation and grid stability problems. And,unfortunately, pyrotechnic current limiters (e.g., fuses) needreplacement after every fault event. Further, while new power electronicFCLs are under development, there are questions about whether they canbe fail-safe and whether they can be extended reliably to transmissionvoltage levels.

To allow HTS cables to survive the flow of fault currents, a significantamount of copper is introduced in conjunction with the HTS wire, butthis adds to the weight and size of the cable. See Development andDemonstration of a Long Length HTS Cable to Operate in the Long IslandPower Authority Transmission Grid by J. F. Maguire, F. Schmidt, S.Bratt, T. E. Welsh, J. Yuan, A. Allais, and F. Hamber, to be publishedin IEEE Transaction on Applied Superconductivity. Often, copper fillsthe central former in the core of the HTS cable around which the HTSwire is helically wound, which prevents the core from being used as apassage for the flow of liquid nitrogen. Alternatively, especially formulti-phase cables, copper wires are mixed in with the HTS wires withinthe helically wound layers of the cable. These copper wires orstructures may be electrically in parallel with the HTS wires and may becalled “copper shunts” within the HTS cable. In the presence of a largefault current that exceeds the critical current of the HTS wires of thecable, they quench or switch to a resistive state that can heat fromresistive I²R losses (where I is the current and R is the resistance ofthe cable). These copper shunts may be designed to absorb and carry thefault current to prevent the HTS wires from over-heating. The amount ofcopper is so large that its total resistance in the cable iscomparatively small and, therefore, has a negligible effect in reducingthe fault current. Copper may be defined to mean pure copper or copperwith a small amount of impurities such that its resistivity iscomparatively low in the 77-90 K temperature range (e.g., <0.5microOhm-cm, or as low as 0.2 microOhm-cm.

In the European SUPERPOLI program (See SUPERPOLI Fault-Current LimitersBased on YBCO-Coated Stainless Steel Tapes by A. Usoskin et al., IEEETrans. on Applied Superconductivity, Vol. 13, No. 2, June 2003, pp.1972-5; Design Performance of a Superconducting Power Link by Paasi etal., IEEE Trans. on Applied Superconductivity, Vol. 11, No. 1, March2001, pp. 1928-31; HTS Materials of AC Current Transport and FaultCurrent Limitation by Verhaege et al., IEEE Trans. on AppliedSuperconductivity, Vol. 11, No. 1, March 2001, pp. 2503-6; and U.S. Pat.No. 5,859,386, entitled “Superconductive Electrical Transmission Line”),superconducting power links were investigated that may also limitcurrent.

Following the typical approach for earlier standalone FCLs, this programinvestigated rigid solid rods or cylinders of HTS material that formedmodules or busbars for the power link. A typical length of a module orbusbar was 50 cm to 2 meters. In a second approach, coated conductorwire was used in which YBCO material was coated on high resistancestainless steel substrates. A gold stabilizer layer was used, but it waskept very thin to keep the resistance per length as high as possible.The wire was helically wound on a rigid cylindrical core which formedanother option for a module or busbar for the power link. In response toa fault current, both these modules switch to a very highly resistivestate to limit the current. The concept proposed in the SUPERPOLIprogram to create a longer length cable was to interconnect the rigidmodules with flexible braided copper interconnections. See U.S. Pat. No.5,859,386, entitled “Superconductive Electrical Transmission Line”. Thepossibility of designing and fabricating a long-length continuouslyflexible cable with fault-current-limiting functionality using lowerresistance and higher heat capacity wires, and hence a lower level oflocal heating, was not considered. Nor was the possibility of additionalgrid elements that could optimize the functionality of the link.

It is desirable to improve the way in which HTS cables handle faultcurrents and to provide an improved alternative to the use of standaloneFCLs or other fault current limiting devices such as highresistance-per-length fault-current limiting modules forming powerlinks. A practical long-length continuously flexible HTS power cablethat incorporates fault current limiting functionality would providemajor benefits in establishing high capacity, low footprint andenvironmentally clean power transmission and distribution, while at thesame time avoiding the necessity for separate and costlyfault-current-limiting devices in crowded utility substations.

SUMMARY OF DISCLOSURE

In a first implementation of this disclosure, a cryogenically-cooled HTScable is configured to be included within a utility power grid having amaximum fault current that would occur in the absence of thecryogenically-cooled HTS cable. The cryogenically-cooled HTS cableincludes a continuous liquid cryogen coolant path for circulating aliquid cryogen. A continuously flexible arrangement of HTS wires has animpedance characteristic that attenuates the maximum fault current by atleast 10%. The continuously flexible arrangement of HTS wires isconfigured to allow the cryogenically-cooled HTS cable to operate,during the occurrence of a maximum fault condition, with a maximumtemperature rise within the HTS wires that is low enough to prevent theformation of gas bubbles within the liquid cryogen.

One or more of the following features may be included. Thecryogenically-cooled HTS cable may include a continuously flexiblewinding support structure. One or more of the HTS wires may bepositioned coaxially with respect to the continuously flexible windingsupport structure. The continuously flexible winding support structuremay include a hollow axial core. The continuously flexible windingsupport structure may include a corrugated stainless steel tube.

A shield layer may be positioned coaxially with respect to continuouslyflexible winding support structure. An insulation layer may bepositioned coaxially with respect to the continuously flexible windingsupport structure and positioned between the one or more conductivelayers and the shield layer. The liquid cryogen may be liquid nitrogen.The liquid nitrogen may be pressurized above atmospheric pressure andmay be subcooled below 77 K. The liquid cryogen may be liquid hydrogen.

The cryogenically-cooled HTS cable may include one or more HTS wires. Atleast one of the HTS wires may be constructed of an HTS material chosenfrom the group consisting of: yttrium or rare-earth-barium-copper-oxide;thallium-barium-calcium-copper-oxide;bismuth-strontium-calcium-copper-oxide;mercury-barium-calcium-copper-oxide; and magnesium diboride. At leastone of the one or more HTS wires may include at least one stabilizerlayer having a total stabilizer thickness within a range of 100-600micrometers and a resistivity within a range of 0.8-15.0 microOhm-cm at90° K. At least one of the one or more HTS wires may include at leastone stabilizer layer having a total stabilizer thickness within a rangeof 200-500 micrometers and a resistivity within a range of 1-10.0microOhm-cm at 90° K. An impedance characteristic and a maximumtemperature rise during a fault condition may be defined by configuringone or more design parameters of one or more of the HTS wires. The oneor more design parameters may include one or more of: a stabilizerresistivity factor; a stabilizer thickness factor; a wire specific heatfactor; and an operating critical current density factor.

One or more high speed switches may be coupled in series with thecryogenically-cooled HTS cable. The one or more high speed switches maybe configured to be opened after the onset of a fault condition. Thecryogenically-cooled HTS cable may be configured to be used in a bus-tieapplication that links a plurality of substations.

In another implementation of this disclosure, a method of configuring acryogenically-cooled HTS cable includes determining a maximum allowableoperating temperature for the cryogenically-cooled HTS cable. Thecryogenically-cooled HTS cable includes a flexible winding supportstructure configured to support one or more conductive layers ofsuperconducting material positioned coaxially with respect to theflexible winding support structure. One or more design parameters of thecryogenically-cooled HTS cable are configured so that, during theoccurrence of a maximum fault condition, an actual operating temperatureof the cryogenically-cooled HTS superconducting cable is maintained at alevel that is less than the maximum allowable operating temperature, andthe maximum fault current is reduced by at least 10%.

One or more of the following features may be included. The maximumallowable operating temperature may essentially correspond to thetemperature at which a refrigerant circulating within at least a portionof the cryogenically-cooled HTS cable changes from a liquid state to agaseous state. The refrigerant may be pressurized liquid nitrogen. Theone or more design parameters may include one or more of: a wireresistivity factor; a stabilizer thickness factor; a specific heatfactor; a fault current duration factor; and a wire operating criticalcurrent per unit width factor. The actual operating temperature of thecryogenically-cooled HTS cable may be determined. The actual operatingtemperature of the cryogenically-cooled HTS cable may be compared to themaximum allowable operating temperature for the cryogenically-cooled HTScable.

Configuring one or more design parameters may include adjusting animpedance of the cryogenically-cooled HTS cable. Adjusting the impedanceof the cryogenically-cooled HTS superconducting cable may include one ormore of: adjusting a length of the cryogenically-cooled HTS cable abovea minimum value; adjusting a resistivity of the cryogenically-cooled HTScable; adjusting a thickness of a stabilizer layer bonded to an HTS wirewithin the cryogenically-cooled HTS cable; adjusting a specific heat ofan HTS wire by means of an encapsulant in the cryogenically-cooled HTScable; and adjusting an operating critical current density of an HTSwire included within the cryogenically-cooled HTS cable.

The stabilizer layer may be constructed, at least in part, of a brassmaterial. The cryogenically-cooled HTS superconducting cable may includeone or more HTS wires. At least one of the HTS wires may be constructedof a material chosen from the group consisting of: yttrium orrare-earth-barium-copper-oxide; thallium-barium-calcium-copper-oxide;bismuth-strontium-calcium-copper-oxide;mercury-barium-calcium-copper-oxide; and magnesium diboride. Thecryogenically-cooled HTS superconducting cable may be coupled to avoltage source having a voltage source impedance. The voltage sourceimpedance of the voltage source may be determined.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will become apparent from the description, the drawings, andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a copper-cored HTS cable systeminstalled within a utility power grid;

FIG. 2 is an isometric view of the copper-cored HTS cable of FIG. 1;

FIG. 3 is an isometric view of a hollow-core HTS cable;

FIG. 4 is a schematic diagram of the hollow-core HTS cable of FIG. 3installed within a utility power grid;

FIG. 5A is a cross-sectional view of an HTS wire;

FIG. 5B is a cross-sectional view of an alternative embodiment HTS wire;

FIG. 6 is a schematic diagram of a utility power grid;

FIG. 7 is a model of the hollow-core HTS cable of FIG. 3 installedwithin a utility power grid; and

FIG. 8 is a flowchart of a method of configuring the hollow-core HTScable of FIG. 3.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Overview

Referring to FIG. 1, a portion of a utility power grid 10 may include ahigh temperature superconductor (HTS) cable 12. HTS cable 12 may behundreds or thousands of meters in length and may provide a relativelyhigh current/low resistance electrical path for the delivery ofelectrical power from generation stations (not shown) or imported fromremote utilities (not shown).

The cross-sectional area of HTS cable 12 may only be a fraction of thecross-sectional area of a conventional copper core cable and may becapable of carrying the same amount of electrical current. As discussedabove, within the same cross-sectional area, an HTS cable may providethree to five times the current-carrying capacity of a conventional ACcable; and up to ten times the current-carrying capacity of aconventional DC cable. As HTS technology matures, these ratios mayincrease.

As will be discussed below in greater detail, HTS cable 12 includes HTSwire, which may be capable of handling as much as one-hundred-fiftytimes the electrical current of similarly-sized copper wire.Accordingly, by using a relatively small quantity of HTS wire (asopposed to a large quantity of copper conductors stranded within thecore of a traditional AC cable), an HTS power cable may be constructedthat is capable of providing three to five times as much electricalpower as an equivalently-sized traditional copper-conductor power cable.

HTS cable 12 may be connected within a transmission grid segment 14 thatcarries voltages at a level of e.g., 138 kV and extends from gridsegment 14 to grid segment 16, which may receive this voltage andtransform it to a lower level of e.g., 69 kV. For example, transmissiongrid segment 14 may receive power at 765 kV (via overhead line or cable18) and may include a 138 kV substation 20. 138 kV substation 20 mayinclude a 765 kV/138 kV transformer (not shown) for stepping down the765 kV power received on cable 18 to 138 kV. This “stepped-down” 138 kVpower may then be provided via e.g., HTS cable 12 to transmission gridsegment 16. Transmission grid segment 16 may include 69 kV substation24, which may include a 138 kV/69 kV transformer (not shown) forstepping down the 138 kV power received via HTS cable 12 to 69 kV power,which may be distributed to e.g., devices 26, 28, 30, 32. Examples ofdevices 26, 28, 30, 32 may include, but are not limited to 34.5 kVsubstations.

The voltage levels discussed above are for illustrative purposes onlyand are not intended to be a limitation of this disclosure. Accordingly,this disclosure is equally applicable to various voltage and currentlevels in both transmission and distribution systems. Likewise, thisdisclosure is equally applicable to non-utility applications such asindustrial power distribution or vehicle power distribution (e.g. ships,trains, aircraft, and spacecraft).

One or more circuit breakers 34, 36 may be connected on e.g., each endof HTS cable 12 and may allow HTS cable 12 to be quickly disconnectedfrom utility power grid 10. Fault management system 38 may provideover-current protection for HTS cable 12 to ensure that HTS cable 12 ismaintained at a temperature that is below the point at which HTS cable12 may be damaged.

Fault management system 38 may provide such over-current protection bymonitoring the current flowing in the segment of the utility grid towhich HTS cable 12 is coupled. For example, fault management system 38may sense the current passing through 138 kV substation 20 (using e.g.,current sensor 40) and may control the operation of breakers 34, 36based, at least in part, on the signal provided by current sensor 40.

In this example, HTS cable 12 may be designed to withstand a faultcurrent as high as 51 kA with a duration of 200 ms (i.e., 12 cycles of60 Hz power). The details of fault management system 38 are described inco-pending U.S. patent application Ser. No. 11/459,167, which was filedon 21 Jul. 2006, and is entitled Fault Management of HTS Power Cable.Typically, in order to withstand this level of fault current, the HTScable may contain a significant amount of copper, which may help tocarry the high fault current and thus protect the HTS wires. The copperis present to protect the HTS cable, but it has no significant currentlimiting effect because of its very low resistance.

Referring also to FIG. 2, there is shown a typical embodiment of asingle-phase copper-cored HTS cable 12 that may include stranded coppercore 100 surrounded in radial succession by first HTS layer 102, secondHTS layer 104, high voltage dielectric insulation layer 106, coppershield layer 108, HTS shield layer 110, coolant passage 112, innercryostat wall 114, thermal insulation 116, vacuum space 118, outercryostat wall 120 and an outer cable sheath 122. HTS layer 102 and HTSlayer 104 may also be referred to as “phase conductors”. Copper shieldlayer 108 may alternatively be positioned on the outside of HTS shieldlayer 110. During operation, a refrigerant or liquid cryogen (e.g.,liquid nitrogen, not shown) may be supplied from an external coolantsource (not shown) and may be circulated within and along the length ofcoolant passage 112. All components of the cable are designed so as toenable flexibility of HTS cable 12. For example, stranded copper core100 (upon which first HTS layer 102 and second HTS layer 104 are wound)is flexible. Accordingly, by utilizing flexible stranded copper core100, an HTS cable 12 is realized that is continuously flexible along itslength. Optionally, a corrugated metal former may be used to support thehelically wound HTS wires, providing continuous flexibility along thelength of the cable.

Additionally/alternatively, additional coaxial HTS and insulation layersmay be utilized. For example, more than two layers of HTS wires may beused for a single phase. Also, three groups of HTS layers separated byinsulation layers (not shown) may be utilized to carry three-phasepower. An example of such a cable arrangement is the Triax HTS Cablearrangement proposed by Ultera (i.e., a joint venture of SouthwireCompany of Carrollton, Ga. and nkt cables of Cologne, Germany). Otherembodiments of HTS cable 12 may include, but are not limited to: warmand/or cold dielectric configurations; single-phase vs. multi-phaseconfigurations; and various shielding configurations (e.g., no shieldand cryostat-based shielding).

Copper core 100 and copper shield layer 108 may be configured to carryfault currents (e.g., fault current 124) that may appear within cable12. For example, when fault current 124 appears within cable 12, thecurrent within HTS layers 102, 104 may dramatically increase to a levelthat exceeds the critical current level (i.e., I_(c)) of HTS layers 102,104, which may cause HTS layers 102, 104 to lose their superconductingcharacteristics (i.e., HTS layers 102, 104 may go “normal”). A typicalvalue for critical current level I_(c) is 4,242 A_(peak) for a cablerated at 3000 A_(rms) (where A_(rms) refers to root-mean-square Amperesof current).

The critical current level in HTS materials may depend upon the choiceof electric field level. Conventionally, the critical current levelI_(c) is defined as an electric field level of 1 microvolt/cm, thoughlower values are also used. However, typical superconductors exhibit atransition region between the zero-resistance (i.e., superconducting)and fully-resistive (i.e., non-superconducting) states as a function ofcurrent level. Conductor losses resulting from operation in thistransition region are below those of the fully-resistive state.Therefore, in practice, portions of conductor in the HTS cable mayswitch to the fully resistive state at a critical current level that isa factor (“f”) times the conventional critical current level I_(c)defined by the 1 microvolt/cm criterion. In meander line wires with YBCOthin films, this factor was determined to be approximately 2, but it wasobserved to vary somewhat with time. See Switching Behavior of YBCO ThinFilm Conductors in Resistive Fault Current Limiters by H. -P. Kraemer etal., IEEE Trans. on Applied Superconductivity, vol. 13, No. 2, June2003, pp. 2044-7. The f-factor for HTS wires with similar YBCO thinfilms is anticipated to be in a similar range (e.g., 1-4).

Accordingly, when the product of the critical current level (as definedabove) and the f-factor is exceeded, the resistance of HTS layers 102,104 may increase significantly and may become comparatively high (i.e.,when compared to copper core 100). As the current passing through aplurality of parallel conductors is distributed inversely with respectto the resistance of the individual conductors, the majority of faultcurrent 124 may be diverted to copper core 100, which is connected inparallel with HTS layers 102, 104. This transmission of fault current124 through copper core 100 may continue until: fault current 124subsides; or the appropriate circuit breakers (e.g., circuit breakers34, 36) interrupt the transmission of fault current 124 through HTScable 12.

Overheating of the HTS conductors in HTS cable 12 may be avoided by twobenefits provided by the copper core 100. First, by redirecting faultcurrent 124 (or at least a portion thereof) from HTS layers 102, 104 tocopper core 100, the overheating of the HTS conductors in HTS cable 12may be avoided. And second the added heat capacity of copper core 100reduces the temperature rise in HTS layers 102 and 104. In the eventthat fault current 124 (or at least a portion thereof) was notredirected from HTS layers 102, 104 to copper core 100, fault current124 may heat the HTS conductors in HTS cable 12 significantly due to thehigh resistance of HTS layers 102, 104, which may result in theformation of gaseous “bubbles” of liquid nitrogen (i.e., due to liquidnitrogen being converted from a liquid state to a gaseous state withincoolant passage 112). Unfortunately, the formation of gaseous “bubbles”of liquid nitrogen may reduce the dielectric strength of the dielectriclayer and may result in voltage breakdown and the destruction of HTScable 12. For warm dielectric cable configurations (not shown), faultcurrent not redirected away from HTS layers 102, 104 may simply overheatand destroy HTS layers 102, 104.

Examples of HTS cable 12 may include but are not limited to HTS cablesavailable from Nexans of Paris France; Sumitomo Electric Industries,Ltd., of Osaka, Japan; and Ultera (i.e., a joint venture of SouthwireCompany of Carrollton, Ga. and NKT cables of Cologne, Germany).

While copper core 100 redirects fault currents (or portions thereof)around HTS layers 102, 104, there are disadvantages to utilizing such an“internal” copper core. For example, copper core 100 may require HTScable 12 to be physically larger and heavier, which may result inincreased cost and greater heat retention within HTS cable 12.Accordingly, more refrigeration may be required to compensate for theadditional heat retention, resulting in higher overall system andoperating costs. Moreover, the increased heat capacity of copper core100, and the thermal resistance between the HTS layers 102, 104, and thecoolant due to the dielectric layer may greatly increase recovery timesshould the energy of a fault current increase the temperature beyond thepoint where superconductivity can be maintained in HTS layers 102, 104.For example, in the event that a fault current is redirected throughcopper core 100, it may take several hours for the refrigeration system(not shown) to cool down HTS cable 12 to within the appropriateoperating temperature range (e.g., 65-77° Kelvin). The time required tocool down HTS cable 12 to within the operating range of the cable iscommonly referred to as the “recovery time”, which may be required byutilities to be as short as possible (e.g. seconds). Alternatively, astandalone fault current limiter may be used with HTS cable 12 to limitfault currents; however this has the disadvantage of requiring anotherlarge and costly piece of electrical equipment to be installed in thesubstation linked to HTS cable 12.

Referring to FIG. 3, there is shown a flexible, hollow-core HTS cable150, according to this disclosure. While HTS cable 150 may includevarious components of prior art copper-cored HTS cable 12, HTS cable 150does not include stranded copper core 100 (FIG. 2), which was replacedwith a flexible hollow core (e.g., inner coolant passage 152). Anexample of inner coolant passage 152 may include, but is not limited to,a flexible, corrugated stainless steel tube. All copper shield layersare removed as well. A refrigerant (e.g., liquid nitrogen) may flowthrough inner coolant passage 152.

In a fashion similar to that of copper-cored HTS cable 12, inner coolantpassage 152 may be surrounded in radial succession by first HTS layer102, second HTS layer 104 (usually helically wound with the oppositehelicity of layer 102), high voltage dielectric insulation layer 106,HTS shield layer 110, coolant passage 112, inner cryostat wall 114,thermal insulation 116, vacuum space 118, outer cryostat wall 120 and anouter cable sheath 122. During operation, a refrigerant (e.g., liquidnitrogen, not shown) may be supplied from an external coolant source(not shown) and may be circulated within and along the length of coolantpassage 112 and inner coolant passage 152. An alternative coolant (e.g.,liquid neon or liquid hydrogen) may be used in the case of lowertransition temperature materials like MgB₂.

As with HTS cable 12, all components of HTS cable 150 are designed so asto enable flexibility continuously along the length of the cable. Forexample and as discussed above, inner coolant passage 152 (upon whichfirst HTS layer 102 and second HTS layer 104 are wound) is flexible.Accordingly, by utilizing flexible inner coolant passage 152, a flexibleHTS cable 150 is realized.

Referring also to FIG. 4, utility power grid portion 10′ may includeflexible, long-length HTS cable 150. Here long-length is defined asgreater than 200 m. It may also include a conventional (i.e.non-superconducting cable, not shown), connected in parallel with HTScable 150. An example of the conventional cable may include but is notlimited to a 500 kcmil, 138 kV Shielded Triple Permashield (TPS) powercable available from The Kerite Company of Seymour, Conn. Theconventional cable may be an existing cable in a retrofit applicationwhere HTS cable 150 is being added to replace one or more conventionalcables to e.g., increase the power capacity of an electrical grid.Alternatively, the conventional cable may be a new conventional cablethat is installed concurrently with HTS cable 150 and interconnectedwith appropriate bus work and circuit breakers.

HTS cable 150 and/or additional HTS cables (not shown) may be includedwithin superconducting electrical path 200, which may include a portionof a utility power grid. Further, superconducting electrical path 200may include other superconducting power distribution devices, such asbuses (not shown), transformers (not shown), fault current limiters (notshown), and substations (not shown).

A fast switch assembly 202 may be coupled in series with HTS cable 150.An example of fast switch assembly 202 is a 138 kV Type PM Power CircuitBreaker manufactured by ABB Inc. of Greensburg, Pa. Fast switch assembly202 (e.g., a switch capable of opening in 4 cycles) may be controllableby fault management system 38. For example, upon sensing fault current124 (FIG. 3), fault management system 38 may open fast switch assembly202, resulting in HTS cable 150 being essentially isolated from faultcurrent 124. For multiphase power, a plurality of fast switch assemblies202 may be utilized. Alternatively, some fast switch assemblies orcircuit breakers are built as a single three-phase device. Fast switchassembly 202 may be reclosed after a time sufficient to allow HTS cable150 to recover to its superconducting state. If existing utility circuitbreakers 34, 36 switch quickly enough to meet the heating requirementsdiscussed below, fast switch assembly 202 may not be required.

The conventional cable (not shown) and/or additional conventional cables(not shown) may be included within a non-superconducting electricalpath, which may include a portion of a power utility grid. Further, thenon-superconducting electrical path may include other power distributiondevices, such as buses (not shown), transformers (not shown), faultcurrent limiters (not shown), and substations (not shown). Thenon-superconducting electrical path may be maintained at a non-cryogenictemperature (e.g., a temperature of at least 273 K, which corresponds to0° C.). For example, the non-superconducting electrical path may not becooled and, therefore, may assume ambient temperature.

As will be discussed below in greater detail, by removing copper core100 (FIG. 2) and copper shield layer 108 (FIG. 2) from the inside of theflexible, long-length HTS cable 150 and by controlling the impedance ofHTS cable 150, HTS cable 150 may be physically smaller, which may resultin decreased fabrication cost and lower heat loss from HTS cable 150.Accordingly, HTS cable 150 may require less refrigeration (when comparedto copper-cored HTS cable 12) and may result in lower overall system andoperating costs. Further, by removing copper core 100 from the inside ofHTS cable 150, the heat capacity of HTS cable 150 and the thermalresistance between HTS layers 102, 104 and the coolant may both bereduced, thus allowing for quicker recovery times in the event thatfault current 124 increase the temperature of HTS cable 150 beyond thepoint where superconductivity may be maintained in HTS layers 102, 104.By removing copper core 100 from the inside of the flexible, long-lengthHTS cable 150 and by controlling the impedance of HTS cable 150, one canincorporate fault current limiting functionality directly into HTS cable150, thus removing the need for a separate standalone fault currentlimiter if one wants to protect the HTS cable or downstream utilityequipment from fault currents.

HTS Cable and Fault Current Limiters

Referring again to FIG. 1, if a fault current within grid section 10causes the current flowing through HTS cable 12 to rise beyond thelimits of conventional circuit breakers 34, 36, an HTS FCL device 42(shown in phantom) or conventional reactor technology (not shown) may beincorporated within grid section 10 to limit the amplitude of the faultcurrent flowing through HTS cable 12 to a level that conventionalcircuit breakers 34, 36 can interrupt. Under normal conditions, whennominal current levels are flowing in grid section 10, HTS FCL device42, which is connected in series with the power flow, may be designed tointroduce very low impedance into the grid (compared to other gridimpedances). However, when a fault current appears in grid section 10,the current causes the superconductor in HTS FCL 42 to instantaneouslygo “normal” or non-superconducting (i.e., resistive), and this adds avery large impedance into grid section 10. HTS FCL 42 may be designed tolimit the fault current to a predetermined level that is within theinterrupting capability of conventional circuit breakers 34, 36.

Standalone HTS FCL devices 42 are being developed by various companies,including American Superconductor Corporation (of Westboro, Mass.) inconjunction with Siemens AG (of Germany). Unfortunately, adding HTS FCLdevice 42 to grid section 10 may be costly and may require a significantamount of space to accommodate device 42, which may be difficult toaccommodate, especially in urban areas. Short busbars or modules withfault current limiting capability are being developed by variouscompanies, including Nexans (of France) and EHTS (of Germany). Whilefault current limiting busbars may have certain applications, they donot provide the sought-after high capacity, low footprint andflexibility that is provided by long-length continuously flexible cablesfor transmission and distribution applications.

According to the present disclosure, an HTS device e.g. continuouslyflexible, long-length HTS cable 150 (FIG. 3), when properly designed,may be used as a fault current limiter itself without the need toincorporate a separate HTS FCL, such as HTS FCL device 42 (FIG. 1). Bycontrolling e.g., the normal-state (resistive) impedance of HTS cable150, the HTS cable itself may be utilized to obtain the desirableeffects (e.g., attenuation of fault currents) of a typical standaloneHTS FCL device (e.g., HTS FCL 42) while avoiding the undesirable effects(e.g., cost and size) of the typical standalone HTS FCL device.Specifically and as will be discussed below in greater detail, if thelength of HTS cable 150 is sufficiently long and if HTS cable 150 ismanufactured to exhibit desired impedance characteristics, continuouslyflexible, long-length HTS cable 150 alone may provide significantattenuation of fault current 124 (FIG. 3) without heating to the pointto create gas bubbles in the liquid cryogen and risking dielectricbreakdown.

Overview of Fault Current Limiting (FCL) HTS Cable and Design of HTSWire for FCL Cable

As will be discussed below in greater detail, by controlling variousparameters of flexible long-length HTS cable 150 (e.g., the electricalresistivity and stabilizer thickness of the HTS wires within cable 150),an HTS cable may be realized that simultaneously 1) provides therequired net resistance to achieve significant reduction of faultcurrent in the cable, and 2) maintains the fault-current-inducedtemperature rise throughout HTS cable 150 at a level that is below amaximum value that prevents the bubbling of the liquid nitrogen coolantcirculating within the cable. As discussed above, the formation ofgaseous “bubbles” of liquid nitrogen may reduce the dielectric strengthof the dielectric layer of HTS cable 150 and may result in voltagebreakdown and the destruction of HTS cable 150.

Electrical resistivity, which may also be known as specific electricalresistance, is a measure of how strongly a material opposes the flow ofelectric current. Specifically, a low electrical resistivity mayindicate a material that readily allows for the movement of electricalcharge. A convenient measure of resistivity is microOhm-cm.

As will be discussed below in greater detail, the structure of HTS cable150 and the design of the HTS wire within HTS cable 150 differfundamentally from the designs that have been proposed for standaloneHTS FCLs or fault-current-limiting busbars.

Referring also to FIG. 5, there is shown a cross-sectional view of oneHTS wire 250 used to construct HTS layers 102, 104 offault-current-limiting HTS cable 150. This wire architecture may also becalled a “coated wire” architecture because a thin layer ofsuperconductor (i.e., an HTS layer) is coated onto a buffered substrate.Typically, the HTS layer comprises the superconductor YBCO, as definedearlier, in particular the composition YBa₂Cu₃O₇ with possiblesubstitutions of rare earth elements for Y. It is understood that theoverall composition may differ from this composition because impurityphases may be present in the layer. Other HTS materials can also be usedin a coated conductor architecture.

In this example, HTS wire 250 used in HTS layers 102, 104 is shown toinclude two stabilizer layers 252, 253 and substrate layer 254. Anexample of substrate layer 254 may include but is not limited tonickel-tungsten, stainless steel and Hastelloy. Positioned betweenstabilizer layer 252 and substrate layer 254 may be buffer layer 256,HTS layer 258 (e.g., an yttrium-barium-copper-oxide—YBCO-layer), and caplayer 260. An example of buffer layer 256 is the combination of yttria,yttria-stabilized zirconia, and cerium oxide (CeO₂), and an example ofcap layer 260 is silver. A solder layer 262 (e.g., a SnPbAg layer) maybe used to bond stabilizer layers 252 and 253 to cap layer 260 andsubstrate layer 254.

In addition to the above-described wire configuration, other wireconfigurations are considered to be included within the scope of thisdisclosure. For example, a single stabilizer layer may be used.Alternatively, a second HTS layer (with its buffer and cap layers, notshown) may be located between second stabilizer layer 253 and theunderside of substrate 254. Optionally, the HTS wire may consist of twostabilizer layers positioned on the outside of the HTS wire, with twosubstrates (each with a buffer layer, an HTS layer, and a cap layer),separated by a third stabilizer layer positioned between the twosubstrate layers. A solder layer may be used to facilitate any of therequired bonds (except possibly between substrate layer 254, bufferlayer 256, HTS layer 258 and cap layer 260).

Referring also to FIG. 5B, there is shown HTS wire 250′, which is analternative embodiment of HTS wire 250. HTS wire 250′ may include asecond substrate layer 280 positioned between second stabilizer layer253 and third stabilizer layer 282. Positioned between stabilizer layer253 (and/or stabilizer layer 282) and substrate layer 280 may be abuffer layer, an HTS layer (e.g., anyttrium-barium-copper-oxide—YBCO-layer), a cap layer, and a solderlayer.

The Stabilizer Layer of HTS Wire

The HTS wire functions most effectively and economically as a faultcurrent limiter if the heat capacity of the HTS wire is high and theelectrical resistivity of the HTS wire is at an optimal level.Stabilizer layer 252 may be essential to achieving these properties.Examples of alloys that may be particularly well suited for stabilizerlayer 252 are low alloy brasses (e.g., Cu—Zn), with e.g., Zn in the3-40% wt range, as well as possibly other brass alloys based on e.g.,the Cu—Sn alloy system. Alloys with resistivities in the 0.8-15micro-ohm cm. range in the 77-110 K temperature range may be optimal.Particular brass alloys may include but are not limited to brass 210 (95Cu-5 Zn), 220 (90 Cu-10 Zn), and 230 (85 Cu-15 Zn), 240 (80 Cu-20 Zn)and 260 (70 Cu-30 Zn). Other copper-based alloys may include e.g., theMonel series (Cu—Ni), which may also provide the above-described rangeof resistivities. Cu—Ni alloys or others with a magnetic transition inthe 70-110 K range may be used and may have the additional advantage ofa large specific heat peak in this temperature range. However, careshould be taken with these alloys to minimize magnetic AC losses byminimizing coercivity.

In order to provide for adequate flexibility in cabling, stabilizerlayer 252, 253 may be in a soft temper state, for example ½ or ¼ hard.The typical total thickness of stabilizer layers 252, 253 of a given HTSwire may be in the 100-600 micrometer range, more preferably in the200-500 micrometer range. If the wires become too thick and rigid, theymay become difficult to strand into the helical winding of acontinuously flexible cable. The thermal conductivity of stabilizerlayer 252, 253 may be greater than 0.1 W/cmK in the 77-110 K temperaturerange to mitigate overheating of the HTS layer (e.g., HTS layers 102,104) during the early stages of a fault and to provide for sufficientlyrapid recovery. Stabilizer layer 252, 253 may be applied by e.g., solderlamination or adhesive bonding. Further, stabilizer layer 252, 253 mayalso be applied by a coating method such as dipping, plating, vapordeposition, electrodeposition, metal-organic liquid-phase deposition orspraying, as either a metal or composite.

Encapsulants for HTS Wire

Additional specific heat may be provided by optionally adding apoorly-conducting “insulator” layer deposited or wrapped around thestabilized HTS wire to encapsulate it. This poorly-conducting insulatorlayer may be referred to as encapsulant 264. Encapsulant 264 may form aliquid-impermeable layer of generally limited heat transfer coefficientto delay heat introduction into the surrounding liquid coolant (e.g.,liquid nitrogen), thus allowing the temperature of the HTS wire tothermalize, i.e., become more uniform across its cross section and thusminimize the occurrence of hot spots and gas bubble formation in theliquid coolant. The surface of the HTS wire may also be optimized (e.g.,with surface features and surface chemistry) to inhibit the onset ofliquid coolant bubbling or boiling.

Encapsulant 264 may be a polymer (e.g., polyethylene, polyester,polypropylene, epoxy, polymethyl methacrylate, polyimides,polytetrafluoroethylene, and polyurethane) that includes commonelectrically insulating materials. The thickness of encapsulant 264 maybe selected to balance the need to cool the HTS wire by heat transferinto the surrounding liquid coolant and the need to maximize thetemperature of the HTS wire without forming gas bubbles within thesurrounding liquid coolant. A general thickness range for encapsulant264 is 25-300 micrometers, and a desirable thickness range forencapsulant 264 is 50-150 micrometers.

In a preferred form, encapsulant 264 may also be weakly electricallyconducting, perhaps through the addition of conducting particles such asmetal, graphite or carbon powder, or may be selected from some of thepartially electrically conducting polymers. The net electricalresistivity of encapsulant 264 may be in the range of 0.0001-100 Ohm cm.While this modest electrical conductivity may not significantly reducethe fault-current-limiting resistance of the HTS wire in its resistiveor normal state, this conductivity may insure that the HTS wires in theHTS cable remain at an equipotential at each cross-section and allow forcurrent sharing between the different HTS wires in HTS cable 150.Maintaining an equipotential is important in case of surges of currentthat may otherwise cause inductively-induced potential differencesbetween the HTS wires, leading to dielectric breakdown and possibledamage to the HTS wires. Optionally, encapsulant 264 may be a highresistivity metal or semiconducting material with resistance in thisrange, or an enamel, glass or crystalline oxide material, which may alsocontain electrical conductivity enhancing materials.

The outer surface of encapsulant 264 may be coated with a material thatdecreases the coefficient of heat transfer between encapsulant 264 andthe surrounding liquid coolant (e.g., liquid nitrogen). Alternatively,the surface of encapsulant 264 may be textured to enhance thecoefficient of heat transfer between encapsulant 264 and the surroundingliquid coolant (e.g., liquid nitrogen). Further, the surface ofencapsulant 264 may be coated with e.g., higher conductivity metalparticles or protruding metals fibers so as to inhibit nucleation byrapidly dissipating heat outward into the surrounding liquid coolant.However, any such surface treatments must also avoid decreasing thedielectric strength in the liquid state.

Encapsulant 264 may be applied using various wrapping/coating methods,including e.g., multi-pass approaches that statistically reduce theincidence of perforations in comparison to single pass approaches.Alternatively, encapsulant 264 may be applied by a coating method suchas dipping, extrusion, plating, vapor deposition or spraying.

Encapsulant 264 may be applied while the HTS wire is in axial tension,up to for example tensile strains in the wire of 0.3% (e.g. of order 100MegaPascals), thus placing encapsulant 264 in a compressed state uponcompletion of the application process, and reducing the likelihood ofperforations in encapsulant 264. Accordingly, once completed,encapsulant 264 may be axially compressed, while the HTS wire withinencapsulant 264 is axially tensioned (when compared to their initialstates).

If encapsulant 264 is applied using a wrapping procedure, an additional,impregnating coating (e.g., a polymer, a paint or a varnish, not shown)may be applied that penetrates any gaps/openings in encapsulant 264 intothe wrapped layers with an impermeable material, thus forming ahermetically-sealed encapsulant. Alternatively, a wrapped encapsulantmay be made hermetic by a rolling or compression process (e.g.,isostatic pressing) that seals the above-referenced gaps/openings.Avoiding gaps or openings is important because liquid cryogenpenetrating towards the metallic stabilizer layers of the wire mayinitiate gas bubble nucleation and boiling during a fault.

Another class of encapsulants or stabilizers are materials that undergoan endothermic phase transition, such as melting or crystal structurephase transition. The use of a material that undergoes such anendothermic phase change at some temperature above the operatingtemperature of the HTS wire (but below the maximum allowable temperatureof the HTS wire) is preferred. An example of an endothermic phase changeis the melting of e.g., low melting temperature organic or inorganicmaterials, that may be added: to encapsulant 264 as discrete embeddedparticles in a composite reinforcement material; as gels/paints that maybe applied to the surface/interfaces of encapsulant 264; or selectivelyto certain regions of encapsulant 264 (e.g., edges, fillets, or ininternal conduit regions). Endothermic phase changes may also includee.g., certain intermetallic phase changes, ordering phase changes, orother second order phase transitions. For example, the material selectedfor encapsulant 264 may melt in the −160° to −70° C. range, with thematerial boiling above approximately −50° C. (with a preferably boilingpoint above ambient temperature), so as to make application ofencapsulant 264 comparatively easy and economical in the liquid orcomposite state (i.e., as a paint, a film coating, an emulsion or agel).

Summary of Wire and Cable Design Criteria

The above-described HTS wire design criteria (i.e., with a thickerstabilizer layer, intermediate values of resistivity, and encapsulants)differ fundamentally from the criteria for prior fault-current-protectedHTS cables, which use first generation HTS wire, and a multifilamentarycomposite with a matrix of high conductivity (<0.5 microOhm-cm in the 77K temperature range) silver. In such prior fault-current-protected HTScables, the goal was to use as high a conductivity material as possiblein the HTS wire or in the HTS cable structure, including large amountsof copper in the cable. The HTS wire design for use in FCL-cables alsodiffers fundamentally from the design criteria for standalone FCLs orthe SUPERPOLI busbars, in which very high resistivity materials wereused and any stabilizer layer is kept as thin as possible to insure ahigh resistance in a short module length. Specifically, for standaloneFCLs or the SUPERPOLI busbars, either bulk superconductors are used(which may have a resistivity of 100 microOhm-cm in the 90-110 Ktemperature range when they are quenched to their normal, resistivestate) or coated conductor wires are used, with high resistancesubstrates like stainless steel. These substrates may have resistivitiesof over 20 microOhm-cm, and in some cases as high as 70 microOhm-cm, inthe 77 K temperature range.

Operation in a Utility Grid

Referring also to FIG. 6, the operation of fault current limiting HTScable 150 within the context of utility power grid 300 is shown. In thisparticular example, utility power grid 300 is shown to include 765 kVbus 302, 69 kV bus 304, and 34.5 kV bus 306. Further, utility power grid300 is shown to include three 138 kV substations 20, 308, 310, each ofwhich provides power to 69 kV bus 304 through three 69 kV substations24, 312, 314. Three 34.5 kV substations 316, 318, 320 may provide powerfrom 69 kV bus 304 to 34.5 kV bus 306. The fault current limiting HTScable 150 is shown coupled between substations 20 and 24.

When a fault current (e.g., fault current 124) is present within utilitypower grid 300, various current components 322, 324, 326, 328, 330, 332(i.e., the portion of fault current 124 passing through HTS cable 150)may flow from all interconnected substations through all available pathsto feed fault current 124, which may appear as a very large load placedon utility power grid 300. When calculating the current componentsrealizable during a fault condition, fault current 124 may be modeled asa short-circuit to ground.

Referring also to FIGS. 7 & 8, when determining how much fault current aparticular substation (e.g., substation 20) contributes to e.g., faultcurrent 124, the open circuit generation voltage may be modeled as idealvoltage source 350. Further, the upstream impedance (i.e., the impedanceseen looking upstream from HTS cable 150) may be combined with thetransformer impedances (i.e., of substation 20) and represented assource impedance 352. Impedance in this context may be a complex vectorquantity consisting of a real and a reactive component. Mathematically,impedance (Z) is equal to R+jX, where R is the real (i.e., resistive)component and X is the reactive (i.e., inductive/capacitive) component.In this example, the reactive component is an inductive impedance andequal to jωL, where ω=2πf and f is the frequency of the current flow(e.g., 60 Hz in North America).

HTS cable 150 is shown terminated to ground 354 because, as discussedabove, fault current 124 is modeled as a short circuit to ground. Ohm'sLaw may be used to determine the expected level of fault current (i.e.,current component 332) provided by substation 20. Using this approachwith respect to the other substations within grid 300, the overall faultcurrent contributions (i.e., the value of e.g., current components 322,324, 326, 328, 330) may be calculated and the fault current componentexpected to pass through HTS cable 150 (i.e., current component 332) maybe determined. Unfortunately, current component 332 may be above thelevel that circuit breakers 34, 36 are capable of handling. Accordingly,HTS cable 150 may be designed to limit this otherwise expected faultcurrent component 332 to a lower, predetermined level that circuitbreakers 34, 36 are capable of handling.

Another important application of the fault current limiting HTS cable isin applications establishing bus-ties within or, more importantly,interconnections between bus-ties in different substations, as shown bythe lines 304 and 306 in FIG. 6. These interconnections allow sharing ofpower between different substations or different transformers withinsubstations depending on the grid loading requirements, while at thesame time maintaining control of fault currents that would otherwisegrow in making such interconnections.

Design of Fault Current Limiting HTS Cable

When designing fault current limiting HTS cable 150, one or more designcharacteristics of HTS cable 150 may be configured so that anytemperature rise (Δ T) that occurs within HTS cable 150 during a faultcurrent is at a level that is below a maximum temperature rise (i.e., ΔT_(max)), as exceeding Δ T_(max) may result in the formation of gaseousnitrogen bubbles. As discussed above, the creation of gaseous nitrogenbubbles may reduce the dielectric strength of the dielectric layer andmay result in voltage breakdown and the damage of HTS cable 150. At thesame time, HTS cable 150 may be designed to be adequately long (i.e.,above a minimum length) to provide adequate resistance to limit thefault current when the HTS wire within HTS cable 150 is driven into itsnormal (i.e., resistive) state.

Accordingly, when designing HTS cable 150, a determination 400 may bemade concerning the maximum allowable operating temperature for e.g.,HTS cable 150. For a liquid nitrogen cooled HTS cable with a pressure of15 bar, the maximum allowable operating temperature is close to 110° K(i.e., the boiling point of liquid nitrogen @ 15 bar). Accordingly, forliquid nitrogen that is subcooled to 72° K, Δ T_(max) is 38° K, or, toprovide some design margin, Δ T_(max) is chosen to be 30° K. These aretypical values for practical HTS cables, but pressures and temperaturerises may vary depending on specific designs.

As discussed above, all cables (both conventional and HTS) attenuatefault current to some degree because all cables have real and reactiveimpedances. However, a typical fault-current-protected HTS cable withlarge amounts of copper has a very low resistive impedance even when theHTS wire is quenched into its normal state. Therefore the reduction ofmaximum fault current due to the resistance of the quenched HTS wire isvery small, perhaps 1% or less, and much less than a minimum level of10% to provide a significant improvement in the operation of a utilitygrid. Additionally and as discussed above, the real and (to a lesserextent) the reactive impedance components in HTS cables (e.g., HTS cable150) may increase several orders of magnitude when the current passingthrough HTS cable 150 exceeds a critical current level (as definedabove). Accordingly, if properly designed to exclude copper and optimizethe resistance of the wire with its stabilizer, HTS cable 150 mayfunction as a fault current limiting device and may attenuate a faultcurrent to a level below several times the superconducting criticalcurrent, thus providing a greater than 10% reduction in the maximumfault current level. In particular, HTS cable 150 may be designed tolimit the fault current to a value of the f-factor (defined above) timesthe critical current.

All significant prior art HTS cable demonstrations to date have includeda significant amount of copper at the cryogenic temperature of thesuperconductor and in close proximity to the superconductor. Therefore,in the event of a fault current that exceeds the critical current level,the majority of the fault current is conducted in the copper, the heatcapacity of the prior art HTS cable is increased, and the temperaturerise within the prior art HTS cable is limited. While this protects theprior art HTS cable from damage, this structure reduces the amplitude ofthe fault current very little due to the large amount of highconductivity, low resistance copper.

With respect to HTS cable 150, the high conductivity copper (and/orother high conductivity metals) are removed and an HTS wire (asdescribed above) is utilized that has a comparatively thick (e.g., totalthickness of 100-600 micrometers, or preferably 200-500 micrometers)stabilizer having a comparatively high resistivity (0.8-15 microOhm-cm,or preferably 1-10 microOhm-cm). The length of HTS cable 150 should belong enough (e.g., typically greater than 200 m) so that the totalresistance of the quenched stabilized HTS cable 150 is large enough toreduce the maximum fault current to approximately a factor f times thecritical current.

Fundamental to the ability to achieve this desired result while, at thesame time, providing flexible and high capacity HTS cable 150 is the useof a coated HTS conductor wire 250 (as described above and asillustrated in FIG. 5). HTS layer 258 should be comparatively thin, andshould include a comparatively thick stabilizer layer 252, 253 (i.e.,typically thicker than HTS layer 258 and substrate layer 254). HTS layer258 should have a high current carrying capacity (e.g., greater than 1Megamp per square centimeter at 77 K). A typical critical current perunit wire width I_(c,w) at the operating temperature is 350 A/cm-width,but values for different wires from different laboratories or commercialmanufacturers can range from 100 A/cm-width to 1000 A/cm-width. Then,when HTS wire 250 switches to a resistive state, the resistance of HTSwire 250 should be comparatively high, resulting in almost all of thecurrent transferring to stabilizer layer 252, 253. HTS wire 150 shouldbe flexible enough to enable helical winding within HTS cable 150. Inpractice, the flexibility requirement may limit the total thickness ofthe combined stabilizer layers 252, 253 to approximately 600micrometers.

For illustrative purposes, let us assume substation 20 is a three-phase13.8 kV substation. Accordingly, the line-to-ground voltage provided bysubstation 20 is 7.97 kV. Further, assume that the unlimited value offault current component 332 is 40 kA and assume an X/R source impedanceratio of 5 (i.e., a typical value). Accordingly, the real (R_(s)) andreactive (X_(s)) impedance values of source impedance 352 may bedetermined 402 to be 0.039+j0.195Ω, as follows: 40 kA=7.97 kV/(R_(s)²+X₂ ²)^(1/2) and X_(s)/R_(s)=5. For this and subsequent calculations,the three-phase system of a given line-to-line voltage (V_(LL)) ismodeled as an equivalent single-phase model using the line-to-groundvoltage (V_(LG)) where V_(LL)=V_(LG)*(3)^(1/2).

For this example, further assume that HTS cable 150 is 1,200 meters inlength (L_(cable)) and is rated at 3,000 amps rms or 3000 A_(rms) (i.e.,I_(rated) in root-mean-square Amperes). As discussed above, innercoolant passage 152 of HTS cable 150 may be surrounded in radialsuccession by first HTS layer 102 and second HTS layer 104. As the wiresof first HTS layer 102 and second HTS layer 104 are helically wrappedaround inner coolant passage 152, the actual length of the individualHTS wires (e.g., HTS wire 250) included within HTS layers 102, 104 arelonger than the length of HTS cable 150. For this example, assume aspiral factor of 1.08, wherein the actual length of the HTS wires are8.00% longer than the length of HTS cable 150.

Additionally, assume that for this example, HTS cable 150 is designed togo normal at 1.6 times I_(rated). This factor may be called atrip-current factor f_(tc). Accordingly, HTS cable 150 may be designedto exhibit superconducting characteristics until 4,800 A_(rms). Thecritical current of the cable is then 4800×1.414=6787 A at its operatingtemperature.

Numerous design parameters may be configured 404 when constructing HTScable 150, examples of which may include but are not limited to: HTSwire width (W); critical current per unit width (I_(c,w)); trip currentfactor f_(tc) f-factor (see below); stabilizer or composite resistivity(ρ); stabilizer or composite thickness (t); conductor specific heat (c);fault current duration (τ); wire count in each phase (N); and cableinductance (X). The total HTS cable critical current may be I_(c,w) WN.By configuring 404 these design parameters, the impedance of HTS cable150 may be adjusted 406 and/or HTS cable 150 may be configured toattenuate a fault current through the HTS cable down to the total cablecritical current times the f-factor, which for typical grid conditionsis much larger than 10% of the original maximum fault current.

HTS Wire Width (W): This design parameter refers to the width of theindividual HTS wires (e.g., HTS wire 250) utilized within HTS layers102, 104. For this example, assume an HTS Wire Width (W) of 0.44 cm, ascommercially available from American Superconductor (344superconductors). This width is primarily determined by the mechanicalrequirements of helically winding the HTS wires around the flexibleformer of a power cable.

Critical Current per Unit Width (I_(c,w)): This design parameter refersto the maximum current level realizable by the individual HTS wires perwidth of the tape-shaped conductor at the standard electric fieldcriterion discussed above. For this example, assume a Critical Currentper Unit Width (I_(c,w)) of 350 Amperes per cm-width (i.e. A/cm-width)at the operating temperature. This parameter is largely determined bythe required rating of the cable and the need to minimize the number (N)of HTS wires used to fabricate the HTS cable.

Trip-Current Factor f_(tc). As discussed above, a typical utility designrequirement is f_(tc)=1.6.

f-Factor (f). This design parameter, first proposed by Kraemer et al.(See Switching Behavior of YBCO Thin Film Conductors in Resistive FaultCurrent Limiters by H. -P. Kraemer et al., IEEE Trans. on AppliedSuperconductivity, vol. 13, No. 2, June 2003, pp. 2044-7) refers to theratio between the current when HTS layers 102, 104 go fully normal orresistive and the critical current. As discussed above and in thisexample, HTS cable 150 goes normal at 4,800 A_(rms) (or about 6,790 Apeak). By multiplying this peak value (i.e., 6,790 A) by the f-factor,the value at which HTS cable 150 is fully normal (i.e.,non-superconducting) may be determined. A first determination done forYBCO thin films by Siemens ((See Switching Behavior of YBCO Thin FilmConductors in Resistive Fault Current Limiters by H. -P. Kraemer et al.,IEEE Trans. on Applied Superconductivity, vol. 13, No. 2, June 2003, pp.2044-7) yielded an f-factor value of approximately 2. This f-factor isexpected to be in the same range for YBCO coated conductor wires (e.g.,a range from 1 to 4). For this and subsequent examples, we assume anf-factor of 2, following the Siemens result. Accordingly and for theabove-described example, we estimate that HTS cable 150 will be fullynormal (i.e., non-superconducting) at about 6,790 Amperes times 2 (i.e.,the f-factor) or 13,580 Amperes. Thus, with a properly configured 408cable (see below), a fault current of 40,000 A_(rms) (56,600 A_(peak))may be limited to 13,580 A_(peak). This represents a reduction of faultcurrent by 76%, significantly larger than the minimum level of 10%needed for useful operational improvement of an electric power grid.

Resistivity (ρ): This design parameter (which may also be known asspecific electrical resistance) is a measure of how strongly a materialopposes the flow of electric current. Typically, resistivity (ρ) is afunction of temperature and may be expressed as ρ_(xx), where “xx”defines the temperature for which the resistivity is calculated. Forthis example, assume a resistivity (ρ₉₀) of 4.0 microOhm-cm attemperature of 90° K, and for simplicity we assume in the estimatesbelow that the temperature dependence in the range from 70 to 110 K maybe ignored. Such a resistivity may be found in e.g., brass. Theconcentration of zinc may be varied to control the resistivity, withhigher resistivities in alloys with more zinc. Many other alloys mayshow similar variations of resistivity with alloy composition; so thereare multiple choices for the stabilizer material.

Stabilizer Thickness (t): This design parameter refers to the thicknessof stabilizer layer 252 included within HTS wire 250. For this example,assume that total stabilizer thickness (t) is approximately 350micrometers. To be more precise, the HTS wire, comprising a substratelayer, superconductor layer, a cap layer, a solder layer, a stabilizerlayer, and an encapsulant, may be a multilayer composite and may becharacterized by the net composite resistivity and thickness of the HTSwire. Since the stabilizer layer is the dominant portion of the wire,the resistivity of the multi-layer composite may be close to theresistivity of the stabilizer layer. However, for simplicity in theestimates below, we assume that in its quenched state current flowsprimarily in the stabilizer layer. Further refinements of this type maybe evident to those skilled in the art.

Specific Heat per volume (C): This design parameter refers to thespecific heat per volume of the composite HTS wire, including substratelayer, HTS layer, cap layer, solder layer and stabilizer layer. For thetypical materials used in the HTS wire, C is close to 2 Joules/cm³K fora temperature of approximately 77 K. For simplicity, we assume thisvalue throughout the temperature range 70-110 K, even though C may varyby 10-20% in this range for certain materials. If HTS wire includes apoorly conducting encapsulant, the encapsulant may add to the specificheat of the wire after several seconds when heat diffusion canthermalize the wire, bringing it to a constant temperature. As a simpleapproximation for the temperature rise calculation below, we canapproximate the effect of the encapsulant by assuming that thecomposite's specific heat is increased by a factor 1+(C_(i)t_(i)/Ct),where the subscript i refers to the encapsulant. In most cases, theencapsulant heat capacity in the 77 K temperature range is also about 2Joules/cm³K, and so for an encapsulant as thick as the composite wire,this factor is 2.

Fault Current Duration (τ): This design parameter refers to the timebefore fast switch assembly 202 or circuit breakers 34, 36 disconnectHTS cable 150 from grid portion 10′. It is desirable to make this timeas short as possible to minimize the energy deposited as heat in thecable, and thus to minimize the heat rise. The fastest switches readilyavailable commercially, along with their sensing circuitry, open in fourcycles (i.e., 67 msec). Thus, the fault current duration is consideredto be 67 msec. If even faster switches become available in the future,it will be desirable to use them.

Wire Count (N): This parameter refers to the total number of wiresincluded within the phase conductor of each phase of the HTS cable.Typically, these are arranged in two HTS layers (e.g., HTS layers 102,104) and are helically wound with the two layers having opposite windingsense (i.e., helicity). For a 3,000 A_(rms) rated cable with 350A/cm-width critical current per width at the operating temperature, alet-through current factor of 1.6, and a wire width of 0.44 cm; therequired conductor count N is 44.

Reactance (X): This design parameter refers to the inductance per unitlength, determined by the amount of magnetic flux produced for a givenelectric current per unit length. For this example, assume an Inductance(X) of 0.017 mH/km, which is characteristic of the Triax cable describedbelow in its supereconducting state.

As substation 20 (in this example) is a three-phase 13.8 kV substation,HTS cable 150 may be a Triax cable (e.g., the Triax HTS Cablearrangement proposed by Ultera, which is a joint venture of SouthwireCompany of Carrollton, Ga. and nkt cables of Cologne, Germany). Each ofthe phases consists of two layers of helical windings, and are allconfigured coaxially and separated by dielectric. The copper strands inthe present Triax cables from Ultera will need to be removed and thewires described above will need to be used to modify the Triax cableinto an FCL-cable.

The resistive component of impedance (Z) of HTS cable 150 in itsquenched state R_(hts(quenched)), shown in FIG. 7, may be calculated asfollows with the parameters given above:

$R_{{hts}{({quenched})}} = \frac{\left( \rho_{90} \right)(L)}{(t)(W)(N)}$$R_{{hts}{({quenched})}} = \frac{\left( {4.0\mspace{14mu}{µ\Omega}\mspace{14mu}{cm}} \right)\left( {1.08 \times 120,000\mspace{14mu}{cm}} \right)}{\left( {\left( {0.0350\mspace{14mu}{cm}} \right)\left( {0.44\mspace{14mu}{cm}} \right)(44)} \right.}$R_(hts(quenched)) = 0.765  Ω

The inductive impedance of the cable is negligible compared to thisrelatively large resistive impedance. Given a specification sheet valueof 0.017 mH/km for a typical cable, one can calculate the equivalentinductance L_(hts) as 0.017 mH/km*1.2 km=0.0204 mH. Reactive impedanceX=jωL, where w=2πf and f is the frequency of the current flow (e.g., 60Hz in North America) which results in X_(hts)=0.00769Ω, which is 100times smaller than R_(hts(quenched)).

Using Ohm's law and the equivalent circuit illustrated in FIG. 7 withthe source impedance 0.039+j0.195Ω as given above, the voltage drop(V_(cable)) across one phase of HTS cable 150 may be calculated usingstandard Kirchhoff's laws to be 7,348 V_(rms). The corresponding rmscurrent (I_(cable)) 356 passing through HTS cable 150 isV_(rms)/R_(hts(quenched))=9,604 A_(rms), which corresponds to a peakcurrent of 9604×1.414 or 13,580 A. Accordingly, current component 332was reduced from 40,000 A_(rms) to 9,604 A_(rms) (i.e., a reduction of76.0%).

As discussed above, the temperature rise (ΔT) that occurs within HTScable 150 during a fault current should be kept at a level that is belowa maximum temperature rise (i.e., ΔT_(max)), as exceeding ΔT_(max) mayresult in the formation of gaseous nitrogen bubbles.

When determining 410 the actual operating temperature of HTS cable 150,the temperature rise (ΔT) realized by HTS cable 150 may be determinedfrom a simple adiabatic calculation, equating the heat generated ρ₉₀ J²τ (where the rms current density J in the quenched superconductor wireis I_(cable)/WNt=fI_(c,w)/√2t) to the heat absorbed C ΔT. From thisrelationship, ΔT can be calculated as follows, using the parametersgiven above:

${\Delta\; T} = \frac{\left. {\left( \rho_{90} \right)\left( I_{cable}^{2} \right)(\tau)} \right)}{\left. \left( {(W)(N)(t)} \right)^{2} \right)(C)}$${\Delta\; T} = \frac{\left( {0.000004\mspace{14mu}\Omega\mspace{14mu}{cm}} \right)\left( {9604\mspace{14mu} A} \right)^{2}\left( {0.067\mspace{14mu}\sec} \right)}{\left( {\left( {0.44\mspace{14mu}{cm}} \right)(44)\left( {0.035\mspace{14mu}{cm}} \right)} \right)^{2}\left( {2\mspace{14mu}{Joules}\text{/}{cm}^{3}K} \right)}$Δ T = 26.9^(^(∘))  K

Accordingly, as the temperature rise (ΔT) realized by HTS cable 150 isless than the maximum allowable temperature rise (ΔT_(max)), gaseousnitrogen bubbles will not be formed, the dielectric strength of thedielectric layer will not be reduced, and HTS cable 150 will not be atrisk of dielectric breakdown leading to permanent damage to the cable.Specifically, for an HTS cable with a pressure of 15 bar, the boilingpoint of liquid nitrogen is 110° K. Accordingly, for a cable operatingwith liquid nitrogen that is subcooled to 72° K, a temperature rise (ΔT) of 26.9° K results in an actual operating temperature of 98.9° K,which is a safe operating temperature when compared 412 to the 110° Kboiling point of liquid nitrogen.

Upon examining the above equation, it becomes clear that increasing thevalues in the denominator reduces temperature rise (ΔT), whileincreasing the values in the numerator increases temperature rise (ΔT).Accordingly, an increase in fault current duration (τ) and/orresistivity (ρ₉₀) may result in an increase in temperature rise (ΔT).Conversely, an increase in stabilizer thickness (t) or specific heat (C)may result in a decrease in temperature rise (Δ T). The wire width W andthe number of wires N are already determined by the practicalrequirements of stranding the cable and the cable rating coupled withthe critical current per width of the wire.

At the same time, the length of the HTS wire in the cable must be longenough to achieve the required resistance. Since a) the maximum limitingcurrent is the f-factor times the wire critical current per widthI_(c,w) times the total width of all wires WN, and b) the resistance isρL/WNt; the minimum length of wire in HTS cable 150 is:L _(min)=(V _(peak))(t)/(f)(I _(c,w))(ρ)  [Equation 1]

With the above values,L _(min)=(1.414×7348 V)(0.035 cm)/(2)(350 A/cm)(0.000004 Ωcm).L_(min)=1,300 m

Taking into account the 1.08 spiral factor, this length corresponds tothe 1200 m cable length originally assumed. Note that for longerlengths, the maximum temperature rise (ΔT) anywhere in the cable willremain at the level calculated above as long as the current is limitedto fI_(c,w)WN. In this case, only portions of the HTS wire will quench,in the manner shown by Siemens (See Switching Behavior of YBCO Thin FilmConductors in Resistive Fault Current Limiters by H. -P. Kraemer et al.,IEEE Trans. on Applied Superconductivity, vol. 13, No. 2, June 2003, pp.2044-7), and the limited current remains at the level fI_(c,w)WN.However, for shorter lengths, the resistance of the HTS wires in thequenched state will decrease, and the current will increase for a givenvoltage according to I=V/R_(hts,quenched). This may lead to greaterheating and increased temperature rise according to the equation for ΔTgiven above. Therefore, the cable length must be greater than thatcalculated above (i.e., 1,300 meters).

Note that the temperature rise may also be calculated as follows:ΔT=ρ(fI _(c,w) /t)²τ/2C  [Equation 2].

From these last two equations, referred to as Equations 1 & 2, one cansee that if one wants to decrease the minimum wire and cable length byincreasing the resistivity ρ or the critical current density I_(c,w) ordecreasing the stabilizer thickness t, temperature rise ΔT willincrease. Alternatively, an increase in the heat capacity through theuse of an encapsulant may decrease the temperature rise. For example,doubling the heat capacity may allow the same temperature rise withtwice the resistivity, and this may reduce the minimum cable length by afactor of two. Note that these equations do not depend on the wire widthW or number of wires N except insofar as they are determined by theoperating rating of the cable and the critical current per width I_(c,w)or the HTS wire.

The conclusion of this cable design analysis is that for applications inwhich all the fault current flows through HTS cable 150, the minimumlength for an FCL HTS cable is in the range of a kilometer for 13.8 kVclass distribution systems. This can be reduced further through e.g.,the use of higher heat capacity as described above. Minimum lengths forother voltages and parameters may be calculated by those skilled in theart from the equations given above or from a more complete analysistaking into account the temperature dependences of all the parameters.

However, if a parallel impedance is provided directly across cable 150(e.g. from breaker 34 to breaker 36 in FIG. 4), the voltage on cable 150may be reduced significantly. For example, we consider a sourceimpedance to be 0.2Ω (inductive) in a 13.8 kV system, corresponding to asingle phase fault current of 40 kA_(rms) in a 13.8 kV_(rms) grid with asingle phase voltage of 8 kV_(rms). A conventional inductive impedanceof 0.046Ω in parallel with HTS cable 150 may reduce the voltage on cable150 to 1500 V_(rms) and give a fault current of 32.5 kA. With thisreduced voltage and the parameters above (including a factor of twoincrease in the heat capacity using encapsulant 264, FIG. 5), thecritical length formula leads to a minimum cable length of about 100 m.Thus, FCL cables may be designed for 13.8 kV grids with lengths as shortas 100 m, provided parallel impedances can be used.

For longer length cables, the resistivity may be decreased. and thetemperature rise correspondingly decreased. This may have the advantageof reducing the recovery time for the cable to return to its originaloperating temperature. For example, for a cable 4.8 km long, theresistivity in the above example may be reduced to 1 microOhm-cm, andthe temperature rise may be reduced from 26.9 K (without encapsulant264, FIG. 5) to 6.7 K.

In the future, faster switch assemblies may become available. In thiscase, the fault duration τ may be decreased and a larger resistivity maybe permitted. For example, with a fault duration of 27 msec, theresistivity may be increased to 10 microOhm-cm, and the minimum lengthof the cable may be decreased (without encapsulant) by a factor of 2.5(10 microOhm-cm divided by 4 microOhm-cm).

Therefore, the concept of an FCL-cable disclosed here may be practicedwith resistivities ranging from 1 to 10 microOhm-cm, and with somefurther adjustment in the parameters considered above, this range couldbe extended to 0.8 to 15 microOhm-cm. However, the low 77 K resistivityof copper (0.2 microOhm-cm) or the high resistivity of stainless steel(50 microOhm-cm) are out of range for a practical continuously flexiblelong-length FCL cable.

Corresponding variations are possible in the parameters of stabilizerthickness t and I_(c,w), though in both cases these may be constrainedby cabling requirements (i.e., the stabilizer cannot get too thick toavoid making the HTS wire too stiff to cable) and by the need to meetutility current ratings.

For transmission level voltages such as 138 kV, a minimum length may beestimated including an encapsulant increasing the heat capacity by afactor of 2 and an increase in the resistivity from 4 to 8 microOhm cm.According to the length formula (described above), the ten-fold increasein the voltage as compared to 13.8 kV class distribution systems,coupled with the two-fold increase in resistivity, implies a minimumlength of (10/2)×1.2 km or 6 km. For transmission level cables, suchlengths are common, showing that the FCL cable design is also possiblein this case.

Another embodiment of this disclosure is an HTS cable that includes morethan one type of HTS wire, for example wire based on the HTS materialBSCCO (bismuth-strontium-calcium-copper-oxide) and wire based on HTSmaterial YBCO (rare earth or yttrium-barium-copper-oxide). Differentsuperconducting materials may have different transition characteristicsfrom superconducting to normal state. For example, YBCO has a muchsharper transition than BSCCO, making it more effective in an FCLapplication, even though both materials have been used in the past(e.g., in the SUPERPOLI program) to demonstrate FCL characteristics. Inthis embodiment, an HTS cable made from BSCCO wire may be designed andoperated to act as a fault current limiting cable by adding anadequately long section of a superconducting cable made from YBCO coatedconductor wire. This may be achieved by splicing in the YBCO section ofcable designed for FCL operation. At normal operating conditions, bothsections are superconducting.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A method of configuring a cryogenically-cooledHTS cable comprising: determining a maximum allowable operatingtemperature for the cryogenically-cooled HTS cable, thecryogenically-cooled HTS cable including a flexible winding supportstructure configured to support one or more conductive layers ofsuperconducting material positioned coaxially with respect to theflexible winding support structure; determining a voltage sourceimpedance; configuring one or more design parameters of thecryogenically-cooled HTS cable so that, during the occurrence of amaximum fault condition, an actual operating temperature of thecryogenically-cooled HTS superconducting cable is maintained at a levelthat is less than the maximum allowable operating temperature, and themaximum fault current is reduced by at least 10%; and adjusting animpedance of the cryogenically-cooled HTS cable.
 2. The method of claim1 wherein the maximum allowable operating temperature essentiallycorresponds to the temperature at which a refrigerant circulating withinat least a portion of the cryogenically-cooled HTS cable changes from aliquid state to a gaseous state.
 3. The method of claim 2 wherein therefrigerant is pressurized liquid nitrogen.
 4. The method of claim 1wherein the one or more design parameters includes one or more of: awire resistivity factor; a stabilizer thickness factor; a specific heatfactor; a fault current duration factor; and a wire operating criticalcurrent per unit width factor.
 5. The method of claim 1 furthercomprising: determining the actual operating temperature of thecryogenically-cooled HTS cable.
 6. The method of claim 5 furthercomprising: comparing the actual operating temperature of thecryogenically-cooled HTS cable to the maximum allowable operatingtemperature for the cryogenically-cooled HTS cable.
 7. The method ofclaim 1 wherein adjusting the impedance of the cryogenically-cooled HTSsuperconducting cable includes one or more of: adjusting a length of thecryogenically-cooled HTS cable above a minimum value; adjusting aresistivity of the cryogenically-cooled HTS cable; adjusting a thicknessof a stabilizer layer bonded to an HTS wire within thecryogenically-cooled HTS cable; adjusting a specific heat of an HTS wireby means of an encapsulant in the cryogenically-cooled HTS cable; andadjusting an operating critical current density of an HTS wire includedwithin the cryogenically-cooled HTS cable.
 8. The method of claim 7wherein the stabilizer layer is constructed, at least in part, of abrass material.
 9. The method of claim 1 wherein thecryogenically-cooled HTS superconducting cable includes one or more HTSwires and wherein at least one of the HTS wires is constructed of amaterial chosen from the group consisting of: yttrium orrare-earth-barium-copper-oxide; thallium-barium-calcium-copper-oxide;bismuth-strontium-calcium-copper-oxide;mercury-barium-calcium-copper-oxide; and magnesium diboride.